My broad research goal is to investigate how cells respond to DNA damage. Accurate and efficient repair is necessary to maintain genomic integrity; loss of this integrity can lead to mutagenesis, human disease, and tumorigenesis. In particular, I am interested in studying the mechanisms of homologous recombination (HR) repair of a particularly deleterious lesion, the DNA double-strand break (DSB). Numerous studies in a variety of model organisms have contributed to the current understanding of HR repair. However, many outstanding questions preclude us from completely understanding the mechanisms of HR and how DNA ends at the site of the DSB are processed in mammalian systems. A two-fold approach will be taken to address these both of these questions. First, using genetic approaches, the mechanisms of gene conversion associated with HR repair of DSBs will be investigated. It is necessary to expand on the gene conversion studies previously reported in this lab and others. A novel repair substrate will allow to more finely map gene conversion tracks. Additionally, analyzing repair events in a mismatch repair defective mutant will uncover heteroduplex DNA (hDNA) that would have otherwise been repaired. This genetic background can be used as a tool to test the following persistent questions regarding mechanisms of gene conversion: one-ended vs. two-ended strand invasion, fate of donor sequence, and hDNA vs. gap repair associated with long gene conversion tracts. Second, a physical analysis of how DNA ends are processed at the site of a DSB and how HR products are formed in mammalian cells will be completed. This analysis will include determining the kinetics of DSB formation, measure rates of DNA degradation at the DSB site, analyze of localization of proteins to the DSB that are required for repair, and lastly, when and how HR repair products are formed. Together, these studies will elucidate the mechanisms of gene conversion, the contribution of the canonical DSBR model and SDSA in the formation of mitotic gene conversion repair products, and clarify the mechanism of end processing of a DSB. [unreadable] [unreadable] The ability for a cell to respond to DNA damage is necessary to maintain genomic integrity. A particularly toxic lesion, DSBs, can arise from environmental factors as well as during normal cellular processes. As evident in the numerous diseases, genome instability, tumorigenesis, and mutagenesis associated with inefficient or inaccurate repair of DSBs, it is integral to understand the mechanisms required in repairing these deleterious lesions. [unreadable] [unreadable] [unreadable]